Polyaniline is one of the most important electrically conducting polymers and is of great interest for commercial applications on a large industrial scale. Thus, the polymer is moderately electrically conductive upon doping with nonoxidizing Bronsted acids. The conductivity depends on the redox state, the doping level, and the moisture content of the polymer.
Although its electronic conductivity is slightly lower than that of other similar materials, such as polypyrrole and polythiophene, polyaniline has the great advantage that it has a high chemical durability against oxygen and moisture. As a result, polyaniline and its derivatives can be used to construct batteries, electronic materials, coatings and devices, molecular electronic biosensors, anticorrosion materials, electroactive and optical materials, etc.
Polyaniline and its derivatives can be prepared by (i) chemical or (ii) electrochemical oxidation of aniline and its derivatives as represented by the following equation: ##STR2## The published literature and patents describe a number of methods for the preparation of polyaniline.
Chemical oxidation of aniline was of great interest to the dye industry in the later 19th and early 20th centuries because of the color of the oxidation product (now called polyaniline). Recently, polyaniline has been rediscovered as an important electrically conducting polymer. Its chemical and electrochemical synthesis have been studied extensively. However, none of the previous methods for the synthesis of electrically conducting polyaniline and its derivatives involves employment of organic initiators as in the present invention.
Chemical synthesis of conducting polyaniline is usually achieved by oxidation of aniline in an acidic aqueous solution (e.g., 1M HCl) using a strong oxidant (e.g., ammonium persulfate (NH.sub.4).sub.2 S.sub.2 O.sub.8)) at about 0.degree.-5.degree. C. If a very pure (e.g., triply distilled) aniline is used, some inorganic catalyst (e.g., FeSO.sub.4) is required to obtain a reasonable rate of the reaction. This catalyst may be an additional source of contamination of the polymer products. A well adopted method for the chemical preparation of conducting polyaniline uses an oxidant aniline molar ratio of about 1:4 and gives a yield of about 10-20%. The conductivity of polyaniline so obtained is about 5-10 S/cm (Siemens/cm=ohm.sup.-1 cm.sup.-1). When weak oxidants (e.g., H.sub.2 O.sub.2) are employed, both the yield and rate of reaction are low, thus limiting the practical commercial applications.
Electrochemical synthesis of conducting polyaniline is generally carried out in an electrolyte consisting of aniline and acid (e.g., 1M HCl) using either potentiostatic or cyclic potential sweep techniques. In order to obtain a reasonable rate of polymerization by both techniques, a high up-limit potential is required, normally 0.8 to 1.2 volts versus saturated calomel electrode (SCE). There are two great disadvantages for the use of the high up-limit potentials. First, more electrical energy is consumed. Second, polyaniline undergoes decomposition at high potentials, and qualities of the polymer decrease.
Accordingly, higher yielding and more effective cost and energy efficient methods of producing polyaniline and its derivatives are needed.